Fire refining of copper consists of charging blister copper and/or copper scrap into a reverberatory furnace and melting the copper. Typically, silica is added as a flux to assist in forming a slag while the molten copper is oxidized by blowing air below the bath surface. Copper oxide is formed and combines with the silica in the formation of the slag. Impurity elements within the copper are removed by (1) oxidation into the slag, (2) solution into the slag and (3) volatilization to the furnace gases. Oxidation into the slag is the principle method of removing Pb and other impurities from the copper. The slag with the impurities contained therein can then be skimmed from the copper bath thereby leaving a purified copper behind. The removal of impurities from the copper bath can be affected by the chemistry of the particular slag employed. A measure of how a particular slag chemistry affects the removal of an individual elemental impurity is the distribution coefficient (percent impurity in the slag divided by the percent impurity in the copper). The distribution coefficient is a well-defined variable at equilibrium and is a function of the thermodynamic variables including the slag composition, temperature and oxygen level. A large distribution coefficient indicates a high level of impurity removal. In developing fluxes for fire refining, it is desirable to find a chemical additive that has a strong affinity for the impurity element or oxide thereof to be removed from the copper. When the source of copper to be refined is scrap obtained from the electrical cable industry, typical major impurities therein are lead, antimony and tin. These elements have a deleterious effect on certain working properties of copper and on its electrical conductivity and their presence in refined copper is generally undesirable. Lead, for example, is generally present in amounts of less than 10 ppm or even &lt;5 ppm in high grade copper employed for copper cable. A method for removing lead and other materials is by repeated slaging of an oxidized copper bath while air is blown through the melt. It is generally preferred to use a slag composition which has the highest distribution ratio for the impurity to be removed. Various slags employed in attempting to remove lead in the prior art include silica sand; iron oxide (Fe.sub.2 O.sub.3)/silica (SiO.sub.2) wherein the percentage of iron oxide is generally 25% or less of the initial slag composition; mixtures of iron oxide, silica, and boric oxide wherein the initial weight ratio of silica to iron oxide is approximately 3:1 and containing from 5 to 25% boric oxide; a mixture of iron oxide, silica and phosphorus pentoxide wherein the initial weight ratio of silica to iron oxide is approximately 3:1 and containing from 5 to 25% phosphorus pentoxide; a pure boric oxide flux; a mixture of cuprous oxide and phosphorus pentoxide or cuprous oxide and boric oxide; and a flux consisting of a mixture of cuprous oxide with silica and iron oxide wherein the ratio of silica to iron oxide is 4:1. I have now discovered the existence of an unexpected peak in the distribution coefficient for the removal of lead impurities from copper when using the iron oxide-silica system. This peak occurs at a iron/silica ratio heretofor not employed in the prior art. I have further discovered that the distribution coefficient can be enhanced by the addition of certain other additives to the flux while operating in the same iron oxide/silica ratio range.